Possible to create caves in terrain?

I’m building an FPS and want to create caves underneath my terrain, but fear this isn’t possible with Unity’s terrain generator. Is there any way to make “holes” in the terrain, underneath which you can stick a cave model or is my only option to model the whole terrain in Blender?

http://answers.unity3d.com/questions/64617/is-it-possible-to-make-holes-in-your-unity-terrain.html

I know this question is old, but I released a new tool on the Asset Store that might be what you’re looking for: Digger - Terrain Caves & Overhangs | Terrain | Unity Asset Store